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Atai was the best friend of mastermind Raban Alou and accompanied him to various meetings as they organised the terror attack.

Curtis Cheng's wife Selina and his son Alpha were present for the sentencing. (AAP)

But it was Alou who handed 15-year-old Farhad Jabar the Smith and Wesson revolver.

Jabar gave the IS salute at Parramatta Mosque then walked to Police Headquarters where he shot Curtis Cheng in the back of the head before he was killed in an exchange of gunfire with guards.

Afterwards when told Mr Cheng was not a police officer, Atai said "he got what he deserved. He's in that environment...aiding them".

The court heard he also gave $1,000 to Jabar's sister to help her escape to Syria in the days before.

Justice Johnson also noted the fortitude, strength and dignity of Mr Cheng's family which he said could be contrasted with the cowardice of those involved.

Supporters of Milad Atai were seen exiting the court after the Islamic State supporter was sentenced to at least 28 years jail. (AAP)

Outside court Mr Cheng's son Alpha said the courts could only do so much.

"Nothing will take away the pain and the trauma that our family has been put through by these really twisted and depraved individuals and regardless of the sentence I don't think this will give them the punishment they deserve."

"We will continue to fight and show our defiance and will not let their actions weaken our spirit and the society we want to live in in Australia."

Mr Cheng's widow Selina stood by his side.

"I say thank you very much for the support from everybody and the understanding, that is so important for me to get healed gradually."